The higher resolution of rendering explains some of it. Advanced completion, syntax highlighting and other productivity features take their share. Some goes to faster to develop programming languages, used to develop these editors, with run-time penalties, coming with this productivity boost.
One can still use Notepad if you want to, but be much less productive with it.
How is it that simple tools like text editors work the same as they did 20 ago but take orders of magnitude more RAM?